AU-717 left channel preamp dead

I need some help. After many years of faithful use my AU-717 has gone on the
fritz. The left channel preamp output is dead.

1) Amp section is fine
2) Switches on front panel are all working fine - R/L audio out of F-2720 board
ok
3) coupling switch on back is working - i am getting
continuity on both sides of the switch when in the connected position
4) power protection relay makes good contact on both right and left

so here is the problem the left channel seems to be
dead. There is a VERY little audio if i crank it up. Intermittantly If i leave it on
for an hour plus it will pop - then power protect off -Then come back on and
then the left channel works.

idont
have a schematic but it seems the problem is on the F-2663 board

Anybody have a clue what i should look for - the
2663 board is a bear to pull out